1. Field of the Invention
The present invention relates to an adjacent carrier frequency interference avoiding method for cellular system and a mobile station. In particular, the present invention relates to an adjacent carrier frequency interference avoiding method for cellular system, by which the occurrence of interference between different sets of signals sent using adjacent carrier frequencies can be avoided, when carrier frequencies used by two separate cellular systems are adjacent to each other on the frequency axis, as well as a mobile station and a base station controller that are used therein
2. Description of the Related Art
A cellular system is generally assigned a specific frequency band, in which multiple different carrier frequencies are set. A mobile station sets a link between itself and a base station that has been set within a service area and communicates with the base station bi-directionally. A transmitter of a base station and that of a mobile station are designed so that, when a base station transmits signals to a mobile station using a certain carrier frequency, or vice versa, power leakage into the frequency band of another carrier frequency that is adjacent on the frequency axis to the carrier frequency used by the transmitter will be minimized. It is impossible, however, for the transmitter to eliminate such power leakage completely, resulting in interference wave power given to signals of an adjacent carrier frequency at a certain ratio.
Each of the mobile stations in one cellular system sets a link between itself and the nearest base station. In uplink, transmitting power from the mobile station is controlled so that the difference among the levels of reception power that the base station receives from multiple mobile stations will not be excessively large. Therefore, the power of interference between adjacent carrier frequencies that is received from signals of an adjacent carrier frequency sent from a mobile station is lower than desired wave power when it reaches the base station.
In downlink, if the level of transmitting power from base station is constant, the interference wave power from signals of a frequency that is adjacent to desired wave signals reaches the mobile station along the same propagation path as desired wave signals. During the travel, the interference wave power attenuates along with desired waves do, and thus the power of interference between adjacent carrier frequencies becomes lower than desired wave power. Therefore, such interference between adjacent carrier frequencies is not a serious problem in one cellular system.
On the other hand, interference between adjacent carrier frequencies may become a serious problem when two cellular systems are in the same service area. More specifically, a serious problem may occur if the first cellular system is assigned a frequency band adjacent to a frequency band of the second cellular system, and the first cellular sets a base station within this service area and communicates with a mobile station of the first cellular system.
This situation will be described using FIG. 1 as one example. As shown in FIG. 1, there are two cellular systems: Cellular Systems A and B. Cellular System A consists of base stations 11 through 13 and a mobile station 21; Cellular System B consists of base stations 61 through 63 and a mobile station 71. As shown in FIG. 2, Cellular System A is assigned carrier frequencies Fa1, Fa2, and Fa3 for uplink, and carrier frequencies Gal, Ga2, and Ga3 for downlink; Cellular System B is assigned carrier frequencies Fb1, Fb2, and Fb3 for uplink and carrier frequencies Gb1, Gb2, and Gb3 for downlink. Carrier frequencies Fa3 and Ga3 of Cellular System A are adjacent to carrier frequencies Fb1 and Gb1 of Cellular System B, respectively, on the frequency axis. The mobile station 21 of Cellular System A is communicating using Fb3 for uplink and Gb3 for downlink; the mobile station 71 of Cellular System B is communicating using Fb1 for uplink, and Gb1 for downlink.
When the mobile station 21 approaches the base station 61, uplink signals of the mobile station 21 will be controlled by the base station 11 to maintain received power at a desired level. At the same time, power received by the base station 61 will grow sharply, and consequently power leakage into the carrier frequency Fb1, which is adjacent to the carrier frequency Fa3, will also become extremely high. This interference between the adjacent carrier frequencies will deteriorate the communication quality of uplink from the mobile station 71 to the base station 61. Similar deterioration in communication quality will take place along the downlink from the mobile station 21, because of an increase in adjacent carrier frequency interference from the carrier frequency Gb1, which is transmitted by the base station 61, to the carrier frequency Ga3.
One method to solve this problem is disclosed in Japanese Patent Laying-Open No. 11-341555. According to this method applied to the examples of FIGS. 1 and 2, the mobile station 21 of Cellular System A will measure the received power Qb of the carrier frequency transmitted from the base station 61 of another cellular system, i.e., Cellular System B. Higher received power Qb means that the base station 61 is nearer. Therefore, if the received power Qb is equal to or lower than a pre-determined threshold, any of the carrier frequencies will be used.
If the received power Qb is higher than the predetermined threshold, carrier frequencies Fa1 or Fa2 for uplink and carrier frequencies Gal or Ga2 for downlink will be used since they are not adjacent to the carrier frequencies of Cellular System B. If Fa3 or Ga3, which is adjacent to the carrier frequencies of Cellular System B, is being used, the carrier frequency will be switched to any of the other carrier frequencies.
When the mobile station 21 controls its transmitting power so that the received power of uplink signals at the base station 11 will be constant, received power, transmitting power, and power of adjacent carrier frequency interference will be in the following relationship. The smaller the received power Qa of the carrier frequency transmitted by the base station 11 of Cellular System A is, the higher the transmitting power from the mobile station 21 becomes. This in turn makes the power of interference between the adjacent carrier frequencies affecting the base station 61 greater. Therefore, another possible method is to have the mobile station 21 measure the received power Qa in addition to the received power Qb, and prevent the carrier frequencies Fa3 or Ga3 from being used if the difference between received power Qb and Qa is greater than the pre-determined threshold.
These methods can reduce interference between adjacent carrier frequencies, because they use carrier frequencies other than Fa3 and Ga3 when the mobile station 21 is likely to cause strong adjacent carrier frequency interference in the uplink from the base station 61 or when the downlink from the mobile station 21 may receive strong adjacent carrier frequency interference from the base station 61.
According to yet another method disclosed in Japanese Patent Laying-Open No. 11-341555, when the mobile station 21 is using Fa3 for uplink, it measures the received power Qb of the carrier frequency transmitted by the base station 61. If the received power Qb is higher than the pre-determined threshold, the mobile station 21 sets the maximum transmitting power for uplink to a value that is lower than the usual maximum value by a difference between the received power Qb and the pre-determined threshold. Another possible method is to have the mobile station 21 additionally measure the received power Qa of the carrier frequency transmitted by the base station 11, and reduce the setting of maximum transmitting power by a difference between the received power Qb and Qa.
These methods can reduce interference between adjacent carrier frequencies, because they reduce the maximum value of transmitting power of the mobile station 21 if the station is using Fa3 for uplink and thus is likely to cause strong adjacent carrier frequency interference in the uplink from the base station 61. These methods are effective when there are no other carrier frequencies available. In addition, since they do not switch from one carrier frequency to another, the duration of strong adjacent carrier frequency interference can be shortened by causing the affecting mobile station to reduce its transmitting power on an autonomous basis.
One problem with the conventional methods described above is that, in order to achieve satisfactory effects, a mobile station in the midst of communication needs to measure frequently the received power of a carrier frequency being used by another cellular system. This measurement is necessary because a mobile station moves while communicating. In the midst of communication, the mobile station may not be in such a state as strong adjacent carrier frequency interference with another system is expected, but may possibly approach a base station of another cellular system.
If a mobile station moves closer to a base station of another system during communication, it must switch to another carrier frequency in a short delay time. In order to do this, the mobile station is required to measure frequently during communication the received power of a carrier frequency being used by another cellular system. This may cause a problem with a mobile station that can receive only one carrier frequency. If it becomes necessary for this type of mobile station to measure the received power of a carrier frequency being used by another cellular system, the mobile station will temporarily become unable to receive signals from the base station with which it is communicating, because it must change the frequency to receive in order to conduct measurement. If the average rate of transmitting information from this type of mobile station is to be maintained at a sufficient level, the base station must increase the rate of transmission by increasing the transmitting power before and after switching to another frequency. This is likely to lead to the increased possibility of interference with other downlink channels and a decreased link capacity.
Furthermore, in the case where the fast closed-loop transmitter power control is conducted, that is, the transmitting power from a mobile station is updated in short cycles via control signals from a base station, the signal power of uplink channels may become inadequate or otherwise excessive because of such interruption of control. This may cause deterioration in communication quality or a decrease in link capacity as a result of increased interference. In addition, power consumption by a mobile station will increase because it must conduct measurement.
An object of the present invention is to resolve the problems described above and provide an adjacent carrier frequency interference avoiding method for cellular system that requires a mobile station to measure less frequently the received power of a carrier frequency being used by another cellular system infrequently, as well as a mobile station and a base station controller that are used therein.
According to the first aspect of the invention, a cellular system comprising at least one base station and at least one mobile station, its adjacent carrier frequency interference avoiding method for use when the cellular system is located in the service area of another cellular system which uses a frequency band adjacent on the frequency axis to the frequency band being used by the cellular system, comprises
the mobile station repeating measurement of the received power of the carrier frequencies being transmitted from the base station of 1 between which and the mobile station a link is set and/or the received power of the carrier frequencies being used by the another cellular system,
assigning carrier frequencies that are not adjacent on the frequency axis to the carrier frequencies being used by the another cellular system to the mobile station according to the results of the measurement, and
changing the frequency of the measurement according to the results of the measurement.
In the preferred construction, the adjacent carrier frequency interference avoiding method for cellular system comprising
assigning to the mobile station carrier frequencies that are not adjacent on the frequency axis to the carrier frequencies of the another cellular system if the difference between received power being transmitted from the base station of 1 and the received power of the carrier frequencies of the another cellular system is greater than the first threshold.
In another preferred construction, the adjacent carrier frequency interference avoiding method for cellular system comprises
changing the frequency of the measurement if the difference between received power being transmitted from the base station of 1 and the received power of the carrier frequencies of the another cellular system is greater than the second threshold.
According to the second aspect of the invention, a cellular system comprising at least one base station and at least one mobile station, its adjacent carrier frequency interference avoiding method for use when the cellular system is located in the service area of another cellular system which uses a frequency band adjacent on the frequency axis to the frequency band being used by the cellular system, comprises
the mobile station repeating measurement of the received power of the carrier frequencies being transmitted from the base station of 1 between which and the mobile station a link is set and/or the received power of the carrier frequencies being used by the another cellular system,
updating the maximum power from the mobile station according to the results of the measurement, and
changing the frequency of the measurement according to the results of the measurement.
In the preferred construction, the adjacent carrier frequency interference avoiding method for cellular system comprises
updating the maximum power from the mobile station if the difference between received power being transmitted from the base station of 1 and the received power of the carrier frequencies of the another cellular system is greater than the first threshold.
In another preferred construction, the adjacent carrier frequency interference avoiding method for cellular system comprises
changing the frequency of the measurement if the difference between received power being transmitted from the base station of 1 and the received power of the carrier frequencies of the another cellular system is greater than the second threshold.
According to the third aspect of the invention, a mobile station in a cellular system which comprises at least one base station and at least one mobile station including the claimed mobile station, the mobile station constituting a cellular system located in the service area of another cellular system which uses a frequency band adjacent on the frequency axis to the frequency band being used by the cellular system, comprises
means for repeating measurement of the received power of the carrier frequencies being transmitted from the base station of 1 and/or the received power of the carrier frequencies being used by the another cellular system,
means for reporting information on the results of the measurement to the base station of 1,
means for changing the carrier frequencies used for communication according to the notification from the base station of 1, and
means for changing the frequency of the measurement.
In the preferred construction, the frequency changing means changes the frequency of measurement according to the results of measurement by the measuring means.
In another preferred construction, the frequency changing means changes the frequency of measurement according to the notification from the base station of 1.
In another preferred construction, the frequency changing means
assigns carrier frequencies that are not adjacent on the frequency axis to the carrier frequencies being used by the another cellular system to the mobile station if the difference between received power being transmitted from the base station of 1 and the received power of the carrier frequencies of the another cellular system is greater than the first threshold.
In another preferred construction, the measurement frequency changing means
changes the frequency of the measurement if the difference between received power being transmitted from the base station of 1 and the received power of the carrier frequencies of the another cellular system is greater than the second threshold.
According to the fourth aspect of the invention, a mobile station in a cellular system which comprises at least one base station and at least one mobile station including the claimed mobile station, the mobile station constituting a cellular system located in the service area of another cellular system which uses a frequency band adjacent on the frequency axis to the frequency band being used by the cellular system, comprises
means for repeating measurement of the received power of the carrier frequencies being transmitted from the base station of 1 and/or the received power of the carrier frequencies being used by the another cellular system,
means for changing the maximum transmitting power according to the results of measurement, and
means for changing the frequency of measurement.
In the preferred construction, the frequency changing means changes the frequency of measurement according to the results of measurement by the measuring means.
In another preferred construction, he mobile station comprises
means for reporting information on the results of the measurement to the base station of 1, and
the frequency changing means changes the frequency of measurement according to the notification from the base station of 1.
In another preferred construction, the maximum power updating means
changes the frequency of the measurement if the difference between received power being transmitted from the base station of 1 and the received power of the carrier frequencies of the another cellular system is greater than the first threshold.
In another preferred construction, the measurement frequency changing means
changes the frequency of the measurement if the difference between received power being transmitted from the base station of 1 and the received power of the carrier frequencies of the another cellular system is greater than the second threshold.
According to another aspect of the invention, a mobile station in a cellular system which comprises at least one base station and at least one mobile station including such mobile station, the mobile station constituting a cellular system located in the service area of another cellular system which uses a frequency band adjacent on the frequency axis to the frequency band being used by the cellular system, comprises
means for repeating measurement of the received power of the carrier frequencies being transmitted from the base station of 1 and/or the received power of the carrier frequencies being used by the another cellular system,
means for reporting information on the results of the measurement to the base station of 1, and
means for changing the maximum transmitting power according to the notification from the base station of 1, and
means for changing the frequency of the measurement.
In the preferred construction, the frequency changing means changes the frequency of measurement according to the results of measurement by the measuring means.
In another preferred construction, the frequency changing means changes the frequency of measurement according to the notification from the base station of 1.
In another preferred construction, the maximum power updating means
changes the frequency of the measurement if the difference between received power being transmitted from the base station of 1 and the received power of the carrier frequencies of the another cellular system is greater than the first threshold.
In another preferred construction, the measurement frequency changing means
changes the frequency of the measurement if the difference between received power being transmitted from the base station of 1 and the received power of the carrier frequencies of the another cellular system is greater than the second threshold.
According to a further aspect of the invention, a base station controller in a cellular system which comprises at least one base station and at least one mobile station including the claimed mobile station, the mobile station being for use in a cellular system located in the service area of another cellular system which uses a frequency band adjacent on the frequency axis to the frequency band being used by the cellular system, comprises
means for receiving a notification of information from the mobile station concerning the received power at the mobile station of the carrier frequency being transmitting from a base station of 1 and/or the received power at the mobile station of the carrier frequency being used by another cellular system,
means for changing the measuring frequency for the received power at the mobile station according to the notification of information from the mobile station concerning the received power in the mobile station of the carrier frequency being transmitting from a base station of 1 and/or the received power in the mobile station of the carrier frequency being used by another cellular system, and
means for transmitting the measuring frequency to the mobile station.
Other objects, features and advantages of the present invention will become clear from the detailed description given herebelow.